T&P shop talk with Steve Rose: GD&T part 2 - true position

Tooling & Production, Oct 1999 by Rose, Steve

Replacing words with symbols happens more and more. Highway traffic signs, road signs for restaurants and restrooms, even icons on the Internet are symbols that replace words. Last month in Shop Talk we began a series discussing Geometric Tolerancing and Dimensioning (GT&D). This is a system that replaces words with symbols on blueprints. Recall this sample print that was introduced and the basic symbols that were defined (datum, feature, specific dimension). This month we will begin discussing the most complex feature on this print, the feature control frame (FCF).

The FCF contains separate pieces of information (notice the frame is divided into sections) that all work together to describe a feature of the part. This FCF describes the position of the hole. It does not describe the size of the hole, the dimension 0.400 0.405 refers to the allowable hole size.

The first section of any FCF contains a GD&sT symbolin this case the true position symbol. The second section gives the tolerance that will be applied to the hole's true position. As we will see, this is a tolerance zone, no tolerances are needed.

A material condition symbol can be applied to the tolerance. Material conditions will be discussed in next month's Shop Talk.

The final symbols in this FCF are the datums, references from which to locate the hole.

The FCF is read like a sentence: "The feature (the hole) is to be located at its true position within a 0.004" tolerance zone to datum A and datum B."

What does this mean? To answer this question, each symbol must be understood on its own.

True position: True position is the exact position of the feature. This is a theoretical position, because remember, no part, program, or person is perfect. So the true position of this hole is X2.25, Y1.1.

Tolerance on true position is a cylindrical tolerance of hole position; this cylindrical tolerance allows the center of the hole to be within a 0.010 diameter zone of the basic dimension.

This diagram shows this zone, the center axis of the hole can fall anywhere in this zone and still be acceptable. This is a cylindrical zone, a tube around the true position.

Tolerancing with true position is superior to straight dimensional tolerancing. Regular tolerance gives an allowance from the exact dimension. Let's look at this simple example, here a 0.25" hole is required and the center location is X2.5, Y2.5. With regular dimensioning, the location allowance is 2.5 +/0.050. As this diagram indicates, the allowance zone is a square, showing that the hole center can vary by 0.005 in +/- X and by 0.005 in +/- Y If the hole center falls anywhere in this square it will be acceptable.

This square has four sides that are 0.010" long. The diagonal of this square can be calculated with Pythagorean's Theorem, and it is found to be 0.014 14" long. If the hole center falls on a diagonal, there is a larger tolerance that will still be acceptable.

If 0.1414" is acceptable in two directions it can be accepted in all directions Oust imagine if the square rotates, the long diagonals move as well). This transforms the square tolerance zone to a circular zone, increasing the area of the zone by 57%.

This increase in tolerance zone is defined by using the true position dimensioning symbol and indicating the size of the diameter. On the sample part, the FCF indicates a cylidrical tolerance zone with a 0.010" diameter.

The tolerance zone given with a true position tolerance is larger then the tolerance zone given with standard dimensioning. This is a 57% increase in tolerance zone!

Steve Rose is a manufacturing consultant and president of RTSI, Solon, OH, which also offers Internet website development. Mr Rose can be reached by, phone 440.542.3066; fax 440.542.3006; e-mail srose@cnc-training.com; or on the web at http://www.cnc-training.com